Fibers consisting of polyesters composed of polyethylene terephthalate or based on polyethylene terephthalate have excellent heat resistance and chemical resistance with high melting point and high modulus of elasticity, and are thus used widely in curtains, carpets, clothing, blankets, sheet texture, table cloths, upholstering texture for chairs, wall materials, artificial hair, interior materials for automobiles, outdoor reinforcing materials and safety nets.
Conventionally, human hair and artificial hair (modacrylic fibers, polyvinyl chloride fibers) have been used in hair articles such as wigs, hairpiece, extension hair, hair bundle and doll hair.
However, it comes difficult to provide human hair, thus increasing the importance of artificial hair. By virtue of flame retardancy, modacrylic fibers are often used as a material of artificial hair but are insufficient in respect of heat resistance. In recent years, fibers based on polyesters represented by polyethylene terephthalate came to be used as fibers for artificial hair because of their excellent tensile strength, heat resistance and good set retention.
Fiber for artificial hair should particularly exhibit suitable matting and color as fiber resembling human hair, in addition to characteristics such as easy setting, setting retention, combing of fiber and less discoloration with light, but the polyester fiber produced by spinning in a usual manner has a flat surface, and the refractive index thereof is as high as 1.72 in the axial direction of the fiber, and is also as high as 1.54 in a vertical direction (diameter direction) to the fiber axis, thus causing strong optical reflection to increase surface gloss, and the fiber cannot be used as artificial hair.
To solve this problem, techniques of matting the surface of polyester fiber have been proposed. For example, the surface of polyester fiber is roughened by forming a large number of fretting mars on the surface of polyester fiber to give artificial matted hair. However, the matted fiber obtained by this method is disadvantageous in that the strength of the fiber is reduced due to the fretting mars formed on the surface of the fiber. In addition, JP-A 63-12716 discloses polyester fiber for artificial hair obtained by treating polyester-based fiber containing fine particles based on silicon oxide, with an aqueous alkali solution to form specific fine protrusions on the surface of the fiber, as well as a method of producing the same.
However, the fiber obtained by this method has a surface with fine and too uniform protrusions and can thus not be sufficiently matted, thus exhibiting strong optical reflection upon receiving an inclined light, and therefore there is a limit to application thereof to artificial hair. When the amount of inorganic fine particles is increased or the size or number of protrusions on the surface of fiber is increased in order to achieve a sufficient matte effect, a large amount of the fine particles are incorporated into the starting polyester. Accordingly, it is said generally and empirically that the strength of spun fiber is deteriorated in proportion to the amount of fine particles contained in the fiber. The strength of artificial hair participates in durability of hairpieces, and the durability is decreased as the strength is reduced.
As a means of providing protrusions without incorporation of inorganic materials influencing the matting of fiber surfaces, a method of irradiating polyester-based fiber with a plasma light is known. However, this method requires approximately 3 steps, that is, a spinning step for forming raw filament, a plasma-treating step for forming protrusions on the outer surface of the raw filament, and a staining step for giving color to artificial hair having concave regions, resulting in low productivity. At present, the treatment time in each step is significantly varied, and thus regulation of each step is not easy, and production control is not easy. As a result, there is a disadvantage that artificial hair having uniform qualities cannot be obtained stably in high productivity.
Fiber from polyester such as polyethylene terephthalate is a combustible material, and is thus poor in flame resistant properties.
Conventionally, various attempts have been made to improve flame resistant properties of polyester fiber. For example, a method of making fiber from polyester comprising phosphorus atom-containing flame-retardant monomers copolymerized therein and a method which involves incorporating a flame retardant into polyester fiber are known.
As the former method which involves copolymerizing flame-retardant monomers, there is proposed a method which involves copolymerizing a phosphorus compound excellent in heat stability containing a phosphorus atom as a ring member (JP-B 55-41610), a method which involves copolymerizing carboxyphosphinic acid (JP-B 53-13479) or a method which involves blending or copolymerizing a phosphorus compound with polyarylate-containing polyester (JP-A 11-124732).
As the latter method which involves incorporating a flame-retardant, there is proposed a method which involves incorporating fine particles of a halogenated cycloalkane compound into polyester fiber (JP-B 3-57990), a method which involves incorporating bromine atom-containing alkylcyclohexane (JP-B 1-24913), a method which involves forming polyester fiber from a polyester-based resin composition containing a phosphorus-based flame-retardant and a triazine-type compound (JP-A 11-335927) or a method of using a composition comprising polymeric polyphosphate incorporated into thermoplastic resin (JP-A 8-120180).
To apply the flame-retardancy techniques described above to artificial hair, polyester fiber comprising a phosphorus compound copolymerized therein has been proposed (JP-A 3-27105, JP-A 5-339805 etc.).
However, the artificial hair requires high flame resistant properties so that the amount of the phosphorus compound copolymerized should be increased in order to use the copolymerized polyester fiber as artificial hair, and as a result, the heat resistance of the polyester is significantly deteriorated thus making melt-spinning difficult, or the polyester may not be ignited or combusted upon approaching flame, but suffers from another problem of melting and dripping.
Significant “extinction by dripping” of the polyester fiber comprising a phosphorus compound copolymerized therein is not preferable for application to artificial hair.
On the other hand, the method of incorporating a flame-retardant into polyester fiber suffers from a problem that the temperature for incorporation of the flame-retardant should be as high as 150° C. or more in order to achieve sufficient flame resistant properties, the treatment time for incorporation should be long, or the flame retardant should be used in a large amount, resulting in problems such as a deterioration in physical properties of the fiber, lower productivity, and higher production costs.
As described above, there has not been obtained suitably matted artificial hair which maintains physical properties of usual polyester fiber, such as heat resistance, strength and elongation and is excellent in flame resistant properties, dripping resistance and setting ability. There is demand for a method of efficiently producing artificial hair at low costs.